From my experience vegans tend to have few or no children. This could be due to the fact that many vegans that I know are young, or the fact that - as said in this answer - vegans tend to have a higher education and be professionals, two things that usually correlate with lower number of children.
Do vegans have less children or is my personal perspective that tells me that? Are there any studies on this subject? (perhaps studies that make hypothesis on the reasons too)
Answering this question is not trivial, as there is no distinct, homogeneous group "the vegans". People become vegan for various reasons.
Beyond that, the process of having children is complex. Diverse reasons may lead to a desire to have children (a scientific approach to social determinants of human reproduction can be found in this article). Also, various factors impact fertility.
There are studies investigating the effect of certain diets on fertility. With regards to a vegan diet, for instance, the impact of soy protein is considered (several studies find no impairing effects, e.g. this or this study). At the same time, there is evidence that "replacing animal sources of protein with vegetable sources of protein may reduce ovulatory infertility risk." However, as with other veg-related studies, the number of studies as well as the representativity of the vegan samples are often limited.
